c16 alcohols and carboxylic acids

Chapter 16Alcohols and Carboxylic Acids

You should be able to:

• Write general and molecular formulae for members of the alcohol homologous series.

• Write fully displayed structures and names of alcohols,

• Identify alcohols by their functional groups,

• Relate the properties of alcohols to their functional groups,

• Describe the reactions of ethanol, and

• Describe the fermentation process by which ethanol is produced from carbohydrates.

Alcohols

• All alcohols belong to a family of organic compounds which contain the –OH group of atoms.

• This group of atoms is called the hydroxyl group. It gives the alcohol its specific chemical properties and is the functional group of alcohols.

• Most alcohols are colourless liquids with a strong smell. They have low boiling and melting points.

• The members of the alcohol homologous series have the following features:

Chapter 16Alcohols and Carboxylic AcidsAlcohols

1. They have the general formula: CnH2n+1OH, where n is an integer.

2. They have a hydroxyl functional group, –OH.3. Their names end with ‘-ol’.4. The formula of each member differs from the

previous one by –CH2.


Name Molecular formula

Relative molecular mass

Structural formula

Methanol CH3OH 32

Ethanol C2H5OH 46

Propanol C3H7OH 60

Butanol C4H9OH 74

Alcohols

• Ethanol is the most important alcohol in the homologous series.

• Only ethanol can be consumed. • Ethanol is a colourless liquid with a strong

smell.

• It has a boiling point of 78 °C and is volatile.

• It mixes readily with water.• It is neutral to litmus.


Properties of Ethanol


Quick Check 1

1. (a) Name an alcohol with three carbon atoms. (b) State its

(i) molecular formula. (ii) structural formula.

2. (a) State the general formula of alcohols. (b) State the formula of an alcohol with 20 carbon atoms.

(c) Do you think it is a liquid, solid or gas at room temperature and pressure?

Solution


Solutions to Quick Check 1

1. (a) Propanol

(b) (i) C3H7OH (ii)

2. (a) CnH2n+1OH

(b) C20H41OH

(c) Solid, its melting point is high because it has

very large molecular mass.

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• When a small piece of sodium is dropped into some ethanol, it reacts steadily to give off bubbles of hydrogen gas leaving behind a colourless solution of sodium ethoxide, CH3CH2ONa.

• On evaporating the solution to dryness, a white sodium ethoxide solid is obtained.

• The reaction is similar to the reaction between water and sodium as in both reactions, the –OH group is replaced by Na+ ion.


Chemical Reactions1. Reaction with a reactive metal like sodium

• Ethanol burns in air (oxygen) to form carbon dioxide and water. The reaction is exothermic and heat energy is given out. Hence, ethanol can be used as a fuel.


2. Combustion

Ethanol + Oxygen Carbon dioxide + WaterC2H5OH(l) + 3O2(g) 2CO2(g) + 3H2O(l)

OH H

• (i) Ethanol is oxidised by hot acidified potassium

dichromate(VI) solution into ethanoic acid:


3. Oxidation to Carboxylic Acids

C2H5OH + 2[O] CH3COOH + H2O

+ 2[O]H+/K2Cr2O7

During the reaction, the potassium dichromate(VI) solution is reduced and the solution turns from orange to green in colour.

H+/K2Cr2O7

+

• (ii) The oxidation of ethanol can also be carried out by using air and bacteria. • When wine, beer or any liquid containing ethanol is left in

the open for a few days, it becomes sour. • Airborne bacteria (called Acetobacter aceti) converts the

alcohol into vinegar, which is a solution of ethanoic acid. • This method is used in the industry to make vinegar.


4. Oxidation to Carboxylic Acids

Ethanol + Oxygen Ethanoic acid + Water C2H5OH +2[O] CH3COOH + H2O

Barrels of vinegar made by oxidation

• Ethanol reacts with carboxylic acids in the presence of concentrated HCl or H2SO4 to form organic compounds called esters. For example, ethanol reacts with ethanoic acid to form an ester called ethyl ethanoate:

• Esters are sweet smelling liquids which can be used for making perfumes and flavouring agents.


5. Formation of Esters

C2H5OH + CH3COOH CH3COOC2H5 + H2O ethanol ethanoic acid ethyl ethanoate

• Ethanol reacts with dehydrating agents such as concentrated sulphuric acid or anhydrous aluminium oxide to form alkenes. This is a dehydration reaction where water is lost across two adjacent carbon atoms in the ethanol.


6. Dehydration to Alkene

In this process, ethanol is heated with excess concentrated sulphuric acid at 170 °C to form ethene. Alternatively, ethanol vapour can be passed over activated alumina (Al2O3) at 450 °C.

O

H H


Quick Check 2

Solution

1. (a) State the products formed by the

combustion of alcohols in oxygen.

(b) Construct a balanced equation for the

combustion of propanol.

2. (a) What is formed when ethanol is

oxidised?

(b) State two different ways in which oxidation

of ethanol can be carried out.


Solutions to Quick Check 2

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1. (a) Carbon dioxide and water

(b) 2C3H7OH(l) + 9O2(g) 6CO2(g) + 8H2O(l)

2. (a) Ethanoic acid

(b) Ethanol can be oxidised

(i) by hot acidified potassium dichromate(VI)

solution (into ethanoic acid) or

(ii) by using air and bacteria.

• Large quantities of ethanol are made for consumption by fermentation.

• Depending on the raw materials used, the ethanol is produced in different kinds of alcoholic drinks such as wine, whisky, brandy, and beer.

• Carbohydrates such as starch and sugar are mixed with water and yeast, a unicellular fungi, and a source of enzymes.

• The mixture is left to ferment without air for a few weeks.

Chapter 16Alcohols and Carboxylic AcidsMaking Ethanol by Fermentation of Carbohydrates

Fermentation is carried out in large tanks

sucrose glucose fructose

Step 1: C12H22O11 + H2O C6H12O6 + C6H12O6

• The temperature of the mixture should be kept around 37 oC for the yeast to work best.

• The reaction only proceeds under anaerobic conditions, i.e. absence of oxygen.

• During fermentation, the yeast feeds on the sugar and changes it into glucose and then into ethanol.

• The ethanol obtained is distilled and made into wine or used as pure ethanol. The carbon dioxide produced in the fermentation is a by-product and can be made into dry ice.


invertase

Step 2: C6H12O6 2C2H5OH + 2CO2

glucose ethanol

zymase

Making Ethanol by Fermentation of Carbohydrates

• We have learned in the previous chapter that alkenes undergo an addition reaction with steam to form alcohols.

• This is the industrial method of making ethanol which is much cheaper than by the fermentation method.

• Ethanol is manufactured by reacting ethene with steam using phosphoric(V) acid as a catalyst at a temperature of about 300 oC.

• The reaction is reversible, and the formation of the ethanol is exothermic.


Making Ethanol from Ethene

CH2=CH2(g) +H2O(g) CH3CH2OH(g) ΔH = –45 kJ/mol

• The table below lists the types of alcoholic drinks and their sources of starch used.


Alcoholic Beverages and Their Sources

Drink Source of starch

% volume of ethanol

Method of preparation

Beer Barley 3 – 8 Fermentation

Wine Grapes / rice 8 – 18

Whisky Barley 30 – 60 Fermentation and

fractional distillation

Brandy Grapes

Vodka Rye / potatoes

• Ethanol is a constituent of alcoholic beverages such as wines and whisky.

• Ethanol is used widely as a solvent for paints, varnishes, liquid soap and other toiletries.

• Ethanol is blended with petrol and used as a fuel for motorcars in many countries.

• Ethanol can be oxidised into ethanoic acid which is used to manufacture plastics and drugs.


Uses of Ethanol


Quick Check 3

Solution

1. What raw materials are required for the production of ethanol by fermentation?

2. Construct chemical equations for the fermentation of sugar.


Solution to Quick Check 3

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1. Carbohydrates2.

Step 1: C12H22O11 + H2O C6H12O6 + C6H12O6

sucrose glucose fructose

Step 2: C6H12O6 2C2H5OH + 2CO2 glucose ethanol

invertase

zymase

• The general formula is CnH2n+1COOH, where n is an integer.• They have an acid functional group:

• Their names end with ‘-oic’, derived from the corresponding alkane by replacing the letter ‘e’ in ‘-ane’ with ‘-oic’.

E.g. methane methanoic acid; ethane ethanoic acid.

• Each member of the homologous series differs from the previous one by an extra –CH2 group.


Carboxylic Acids

• The molecular model and structural formula of a typical carboxylic acid, such as ethanoic acid, is as shown:


Structure of Ethanoic acid

Structure of ethanoic acid

Name Molecular formula

Structural formula

Methanoic acid HCOOH

Ethanoic acid CH3COOH

Propanoic acid C2H5COOH

Butanoic acid C3H7COOH

• It is a colourless liquid with a strong sour smell.• It mixes readily with water and is commonly called acetic

acid.• It turns blue litmus red.

• Carboxylic acids are weak monobasic acids. • Carboxylic acids show typical reactions of mineral acids.

They react less vigorously as they are weak acids and dissociate only partially in water to form hydrogen ions.


Properties of Ethanoic Acid

Chemical reactions

1. Reaction with Metals

Ethanoic acid reacts with more reactive metals like sodium, calcium and magnesium to form a salt and liberate hydrogen gas.

2Na(s) + 2CH3COOH(aq) 2CH3COONa(aq) + H2(g)

• Ethanoic acid reacts with the oxides and hydroxides of metals to form salt and water. Here, the reaction is a neutralisation reaction.

• Ethanoic acid reacts with carbonates to produce salt, water and carbon dioxide.


2. Reaction with Metallic Oxides and Hydroxides

MgO(s) + 2CH3COOH(aq) (CH3COO)2Mg(aq) + H2O(l)NaOH(aq) + CH3COOH(aq) CH3COONa(aq) + H2O(l)

3. Reaction with Carbonates

2CH3COOH(aq) + Na2CO3(s) 2CH3COONa(aq) + H2O(l) + CO2(g)CH3COOH(aq) + NaHCO3(s) CH3COONa(aq) + H2O(l) + CO2(g)

• Ethanoic acid reacts with alcohols to form sweet smelling compounds called esters.

• Concentrated sulphuric acid is added as a catalyst. The process of forming esters is called esterification.


4. Reaction with Alcohols

The formation of the esters from different carboxylic acids and alcohols:

CH3COOH + C2H5OH CH3COOC2H5 + H2Oethanoic acid ethanol ethyl ethanoate

CH3COOH + CH3OH CH3COOCH3 + H2Oethanoic acid methanol methyl ethanoate

C2H5COOH + C2H5OH C2H5COOC2H5 + H2Opropanoic acid ethanol ethyl propanoate

• Ethanoic acid is a very important chemical used in manufacturing a wide variety of things.

• Annual production of ethanoic acid in the world is estimated to be more than 6 million tonnes.

• There is a wide range of products made from ethanoic acid. This includes:

– making various kinds of plastics such as acetates for making fabric, films, adhesives and paints.

– making drugs like aspirin,– making bleaching agents in detergents; chemicals like

herbicides and dyes, and– vinegar, which is used in cooking, as an additive to chilli and

tomato sauces and for preserving vegetables.


Uses of Ethanoic Acid

Esters are organic compounds with characteristic fruity smells.

• They are used in food flavouring and in making perfumes.

• Esters can be prepared by reacting an alcohol with a carboxylic acid using concentrated sulphuric acid as the catalyst or dehydrating agent.


Esters

E.g. Ethyl ethanoate can be prepared by reacting ethanoic acid with ethanol. In this case, the hydrogen in the –COOH group has been replaced by an ethyl group. The formula of ethyl ethanoate is:

• Short-chain esters are more volatile than carboxylic acids of the same number of carbons.

• They are not able to form hydrogen bonds with themselves due to the absence of hydroxyl group.

• Thus the boiling points of esters are usually lower than that of carboxylic acids of the same number of carbon atoms.

• Short-chain esters are fairly soluble in water but solubility decreases with increasing chain length.

• Esters are soluble in water because they are able to form hydrogen bonds with water molecules.

• The slightly positive end of a water molecule can form a hydrogen bond with the lone pair of electrons on one of the oxygen atoms in an ester.


Physical properties of esters

Solubility of esters in water

Boiling points

• The ester linkage can be broken by addition of water. • The process is therefore known as hydrolysis. • Hydrolysing esters using only water is very slow. • The reaction is thus catalysed by using dilute acid like dilute

hydrochloric acid or dilute sulphuric acid. • The ester is heated under reflux with a dilute acid and the

products are separated by fractional distillation.


Chemical properties of esters

(a) Hydrolysis of ethyl ethanoate:

(b) Hydrolysis of methyl propanoate:

• Alkalis can also be used. When an alkali is used, the salt of the acid, instead of the acid itself, is formed.

• Hydrolysis of esters using an alkali is sometimes known as saponification. (used in producing soap)

• E.g. In the hydrolysis of ethyl ethanoate using sodium hydroxide solution, sodium ethanoate instead of ethanoic acid is formed.


Alternative method

CH3COOCH2CH3(aq) + NaOH(aq) CH3COO-Na+(aq) + CH2CH3OH(aq)


Quick Check 4

Solution

1. (i) What is the general formula of a carboxylic acid? (ii) State the formula of a carboxylic acid with 10 carbon atoms.2. State the name and write the structural formula of a

carboxylic acid with: (a) 2 carbon atoms, (b) 3 carbon atoms and (c) 4 carbon atoms.3. (a) What organic compound is formed when ethanoic acid reacts with ethanol? (b) What name is given to this type of reaction and what is the

catalyst required? (c) Construct an equation to show the reaction between ethanoic acid and ethanol.

1. (i) CnH2n+1COOH (ii) C10H21COOH

2. (a) ethanoic acid (b) propanoic acid (c) butanoic acid

3. (a) Ethyl ethanoate (b) Esterification, concentrated sulphuric acid conc. H2SO4

(c) CH3COOH + C2H5OH CH3COOC2H5 + H2O ethanoic acid ethanol ethyl ethanoate


Solution to Quick Check 4

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References

• Chemistry for CSEC Examinations by Mike Taylor and Tania Chung

• Longman Chemistry for CSEC by Jim Clark and Ray Oliver

c16 alcohols and carboxylic acids

Science

properties of alcohols

names of alcohols

functional group of

alcoholsall alcohols

general formula of alcohols

carboxylic acidssolutions

carboxylic acidsalcoholsthey

carboxylic acids2